The present invention concerns bone anchors, particularly useful for engagement in the vertebrae. In a particular embodiment, the invention contemplates a bone screw assembly that allows loading of an elongated rod or other member extending along the spine from the side.
Several techniques and systems have been developed for correcting and stabilizing the spine and for facilitating fusion at various levels of the spine. In one type of system, a bendable rod is disposed longitudinally along the length of the spine or vertebral column. The rod may be preferably bent to correspond to the normal curvature of the spine in the particular region being instrumented. For example, the rod can be bent to form a normal kyphotic curvature for the thoracic region of the spine, or a lordotic curvature for the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along the length of the spinal column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra. For instance, one such fixation element is a hook that is configured to engage the laminae of the vertebra. Another very prevalent fixation element is a spinal screw which can be threaded into various aspects of the vertebral bone.
In one typical procedure utilizing a bendable rod, the rod is situated on one or opposite sides of the spine or spinous processes. A plurality of bone screws are threaded into a portion of several vertebral bodies, for example into the pedicles of these vertebrae. The rod(s) are connected or affixed to these plurality of bone screws to apply corrective and stabilizing forces to the spine.
One example of a rod-type spinal fixation system is the TSRH® Spinal System sold by Medtronic Sofamor Danek, Inc. The TSRH® System includes elongated rods and a variety of hooks, screws and bolts all configured to create a segmental construct throughout the spine. In one aspect of the TSRH® System, the spinal rod is connected to the various vertebral fixation elements by way of an eyebolt. In this configuration, the fixation elements are engaged to the spinal rod laterally adjacent to the rod. In another aspect of the TSRH® System, a variable angle screw is engaged to the spinal rod by way of an eyebolt. The variable angle screw allows pivoting of the bone screw in a single plane that is parallel to the plane of the spinal rod. Details of this variable angle screw can be found in U.S. Pat. No. 5,261,909 to Sutterlin et al., owned by the Assignee of the present invention. One goal achieved by the TSRH® System is that the surgeon can apply vertebral fixation elements, such as a spinal hook or a bone screw, to the spine in appropriate anatomic positions. The TSRH® System also allows the surgeon to easily engage a bent spinal rod to each of the fixation elements for final tightening.
Another rod-type fixation system is the Cotrel-Dubosset/CD Spinal System sold by Medtronic Sofamor Danek, Inc. Like the TSRH® System, the CD® System provides a variety of fixation elements for engagement between an elongated rod and the spine. In one aspect of the CD® System, the fixation elements themselves include a body that defines a slot within which the spinal rod is received. The slot includes a threaded bore into which a threaded plug is engaged to clamp the rod within the body of the fixation element. The CD® System includes hooks and bone screws with this “open-back” configuration. Details of this technology can be found in U.S. Pat. No. 5,005,562 to Cotrel. One benefit of this feature of the CD® System is that the fixation element is positioned directly beneath the elongated rod. This helps reduce the overall bulkiness of the implant construct and minimizes the trauma to surrounding tissue.
These and other vertebral anchors have channels for an elongated rod or other member that open upward, i.e. directly away from the bone to which the anchor is attached. The convenience of such a structure is clear, as the anchor can be first placed in the bone, then the rod can be essentially lain on top of it, within the channel. In many cases, however, a surgeon may wish to use anchors to translate the vertebral body to the rod. This translation will typically involve horizontal as well as vertical components. Side loading implants, along with their associated instruments, can simplify the execution of this type of maneuver. Use of such implants can present less interference from lateral tissue, the potential to pre-load the locking components prior to inserting the anchor and also innovative means of provisionally capturing the rod prior to final tightening.
To address these issues, bone anchors having a channel opening to the side have been developed. However, new and improved side-loading bone anchors are still needed in the industry.